Silicon carbide is expected as a next-generation power semiconductor material. An epitaxial layer made to grow on a silicon carbide substrate includes, specifically, a drift layer which becomes a breakdown-voltage holding layer of a power device. In the case of N-type silicon carbide, the power device silicon carbide substrate generally has a carrier concentration on the order of 1018 to 1019 cm−3. In contrast, the drift layer has a carrier concentration on the order of 1015 to 1016 cm−3. Therefore, for power device applications, the silicon carbide substrate normally has a carrier concentration about 10 times to 1000 times higher than the carrier concentration of the drift layer. A lattice constant of silicon carbide is dependent on its carrier concentration. More specifically, the higher the carrier concentration, the smaller the lattice constant becomes. Due to a difference in carrier concentration, compression stress is applied to the epitaxial layer at the interface between the epitaxial layer such as the drift layer and the silicon carbide substrate. The stress may cause a dislocation or crystal defect failure in the epitaxial layer. Degradation of crystal quality may hamper carrier movement, causing deterioration of device characteristics.
As a solution to the above-described problem, a technique is known which causes a buffer layer to epitaxially grow on the silicon carbide substrate and then causes the drift layer to epitaxially grow on the buffer layer. Thus, a buffer layer having an intermediate carrier concentration can be provided between the silicon carbide substrate and the drift layer. For example, paragraph 0050 of JP 2000-319099 A discloses a technique which changes the carrier concentration of the buffer layer in a step form or provides a continuous and linear gradient in the carrier concentration of the buffer layer.